Browsing by Author "Xiao, John Q."
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Item Bimetal–organic frameworks derived tuneable Co nanoparticles embedded in porous nitrogen-doped carbon nanorods as high-performance electromagnetic wave absorption materials(Journal of Materials Chemistry C, 2021-05-04) Pan, Jiannan; Yang, Huadong; Hong, Qu; Wen, Hui-Min; Xiao, John Q.; Hu, JunThe in situ pyrolysis of metal–organic-frameworks (MOFs) is an effective strategy to prepare magnetic metal nanoparticle (NP) doped porous carbon materials. These composite materials have shown great potential as high performance electromagnetic wave (EMW) absorption materials. So far, the precise control of composite composition and structure has remained a major challenge in constructing highly porous composites with uniformly distributed NPs. In this work, we report a facile route to synthesize tuneable Co NPs embedded in porous nitrogen-doped carbon (Co/NC) nanorods through the direct thermolysis of the bimetal–organic framework (CoZn–ZIF) precursor. By adjusting the proportion of Co2+ in the MOF precursor, the content and distribution of Co NPs in the composite absorber change accordingly. When the molar ratio between Co2+ and Zn2+ is 3 : 1, the carbonized composites exhibit the largest external surface area and the best EMW absorption performance. With a filler mass loading of merely 15 wt%, the minimum reflection loss (RLmin) reaches −52.3 dB at 10.1 GHz with a thin layer thickness of 2.5 mm. The largest effective absorption bandwidth (EAB) of 5.0 GHz (11.1–16.1 GHz) is achieved in a 2.0 mm thick sample. The qualified bandwidth can be up to 14.5 GHz (3.5–18.0 GHz) with the integrated thickness from 1.0 mm to 5.5 mm. The enhanced conductive/magnetic losses, strong interfacial/dipolar/defect polarization, hierarchical pore structure and the geometric effect endow the Co/NC absorber with improved impedance matching and enhanced attenuation of EMW. This work provides a good direction for the future study of MOF-derived lightweight and efficient EMW absorbing materials.Item Direct probing of strong magnon–photon coupling in a planar geometry(Quantum Science and Technology, 2022-10-31) Kaffash, Mojtaba T.; Wagle, Dinesh; Rai, Anish; Meyer, Thomas; Xiao, John Q.; Jungfleisch, M. BenjaminWe demonstrate direct probing of strong magnon–photon coupling using Brillouin light scattering (BLS) spectroscopy in a planar geometry. The magnonic hybrid system comprises a split-ring resonator loaded with epitaxial yttrium iron garnet thin films of 200 nm and 2.46 μm thickness. The BLS measurements are combined with microwave spectroscopy measurements where both biasing magnetic field and microwave excitation frequency are varied. The cooperativity for the 200 nm-thick YIG films is 1.1, and larger cooperativity of 29.1 is found for the 2.46 μm-thick YIG film. We show that BLS is advantageous for probing the magnonic character of magnon–photon polaritons, while microwave absorption is more sensitive to the photonic character of the hybrid excitation. A miniaturized, planar device design is imperative for the potential integration of magnonic hybrid systems in future coherent information technologies, and our results are a first stepping stone in this regard. Furthermore, successfully detecting the magnonic hybrid excitation by BLS is an essential step for the up-conversion of quantum signals from the microwave to the optical regime in hybrid quantum systems.Item High-performance HZO/InAlN/GaN MISHEMTs for Ka-band application(Semiconductor Science and Technology, 2023-02-01) Cui, Peng; Moser, Neil; Chen, Hang; Xiao, John Q.; Chabak, Kelson D.; Zeng, YupingThis paper reports on the demonstration of microwave power performance at 30 GHz on InAlN/GaN metal–insulator–semiconductor high electron mobility transistor (MISHEMT) on silicon substrate by using the Hf0.5Zr0.5O2 (HZO) as a gate dielectric. Compared with Schottky gate HEMT, the MISHEMT with a gate length (LG) of 50 nm presents a significantly enhanced performance with an ON/OFF current ratio (ION/IOFF) of 9.3 × 107, a subthreshold swing of 130 mV dec−1, a low drain-induced barrier lowing of 45 mV V−1, and a breakdown voltage of 35 V. RF characterizations reveal a current gain cutoff frequency (fT) of 155 GHz and a maximum oscillation frequency (fmax) of 250 GHz, resulting in high (fT × fmax)1/2 of 197 GHz and the record high Johnson's figure-of-merit (JFOM = fT × BV) of 5.4 THz V among the reported GaN MISHEMTs on Si. The power performance at 30 GHz exhibits a maximum output power of 1.36 W mm−1, a maximum power gain of 12.3 dB, and a peak power-added efficiency of 21%, demonstrating the great potential of HZO/InAlN/GaN MISHEMTs for the Ka-band application.Item Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures(Physical Review B, 2022-10-10) Wang, Yang; Mambakkam, Sivakumar V.; Huang, Yue-Xin; Wang, Yong; Ji, Yi; Xiao, Cong; Yang, Shengyuan A.; Law, Stephanie A.; Xiao, John Q.Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of the nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.Item Second-harmonic signature of chiral spin structures in W/Pt/Co heterostructures with tunable magnetic anisotropy(Journal of Physics D: Applied Physics, 2023-03-28) Wang, Yang; Chan, Ying-Ting; Wang, Xiao; Wang, Tao; Cheng, Xuemei M.; Wu, Weida; Xiao, John Q.Second-harmonic Hall voltage (SHV) measurement method has been widely used to characterize the strengths of spin–orbit torques (SOTs) in heavy metal/ferromagnet thin films saturated in the single-domain regime. Here, we show that the magnetic anisotropy of a W/Pt/Co trilayer can be robustly tuned from in-plane to out-of-plane by varying W, Pt, or Co thicknesses. Moreover, in samples with easy-cone anisotropy, SHV measurements exhibit anomalous 'humps' in the multidomain regime accessed by applying a nearly out-of-plane external magnetic field. These hump features can only be explained as a result of the formation of Néel-type domain walls, efficiently driven by nevertheless small SOTs in this double heavy metal heterostructure with canceling spin Hall angles.Item Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn(Physical Review Materials, 2023-03-10) Wang, Xinhao; Hossain, Mohammad Tomal; Thapaliya, T. R.; Khadka, Durga; Lendinez, Sergi; Chen, Hang; Doty, Matthew F.; Jungfleisch, M. Benjamin; Huang, S. X.; Fan, Xin; Xiao, John Q.There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in Mn3Sn, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the Mn3Sn crystal group.Item A subwavelength resolution microwave/6.3 GHz camera based on a metamaterial absorber(Nature Publishing Group, 2017-01-10) Xie, Yunsong; Fan, Xin; Chen, Yunpeng; Wilson ., Jeffrey D; Simons, Rainee N; Xiao, John Q.; Yunsong Xie; Xin Fan; Yunpeng Chen; Jeffrey D.Wilson; Rainee N. Simons; John Q. Xiao; Xie, Yunsong; Chen, Yunpeng; Xiao, John QThe design, fabrication and characterization of a novel metamaterial absorber based camera with subwavelength spatial resolution are investigated. The proposed camera is featured with simple and lightweight design, easy portability, low cost, high resolution and sensitivity, and minimal image interference or distortion to the original field distribution. The imaging capability of the proposed camera was characterized in both near field and far field ranges. The experimental and simulated near field images both reveal that the camera produces qualitatively accurate images with negligible distortion to the original field distribution. The far field demonstration was done by coupling the designed camera with a microwave convex lens. The far field results further demonstrate that the camera can capture quantitatively accurate electromagnetic wave distribution in the diffraction limit. The proposed camera can be used in application such as non-destructive image and beam direction tracer.